Overexpression of PIP2;5 aquaporin alleviates effects of low root temperature on cell hydraulic conductivity and growth in Arabidopsis

The effects of low root temperature on growth and root cell water transport were compared between wild-type Arabidopsis (Arabidopsis thaliana) and plants overexpressing plasma membrane intrinsic protein 1;4 (PIP1;4) and PIP2;5. Descending root temperature from 25°C to 10°C quickly reduced cell hydraulic conductivity (L(p)) in wild-type plants but did not affect L(p) in plants overexpressing PIP1;4 and PIP2;5. Similarly, when the roots of wild-type plants were exposed to 10°C for 1 d, L(p) was lower compared with 25°C. However, there was no effect of low root temperature on L(p) in PIP1;4- and PIP2;5-overexpressing plants after 1 d of treatment. When the roots were exposed to 10°C for 5 d, L(p) was reduced in wild-type plants and in plants overexpressing PIP1;4, whereas there was still no effect in PIP2;5-overexpressing plants. These results suggest that the gating mechanism in PIP1;4 may be more sensitive to prolonged low temperature compared with PIP2;5. The reduction of L(p) at 10°C in roots of wild-type plants was partly restored to the preexposure level by 5 mm Ca(NO(3))(2) and protein phosphatase inhibitors (75 nm okadaic acid or 1 μm Na(3)VO(4)), suggesting that aquaporin phosphorylation/dephosphorylation processes were involved in this response. The temperature sensitivity of cell water transport in roots was reflected by a reduction in shoot and root growth rates in the wild-type and PIP1;4-overexpressing plants exposed to 10°C root temperature for 5 d. However, low root temperature had no effect on growth in plants overexpressing PIP2;5. These results provide strong evidence for a link between growth at low root temperature and aquaporin-mediated root water transport in Arabidopsis.     

Arabidopsis phosphatidylinositol 4-phosphate 5-kinase genes PIP5K7, PIP5K8, and PIP5K9 are redundantly involved in root growth adaptation to osmotic stress

Phosphatidylinositol 4-phosphate 5-kinase (PIP5K) produces phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P₂), a signaling phospholipid critical for various cellular processes in eukaryotes. The Arabidopsis thaliana genome encodes 11 PIP5K genes. Of these, three type B PIP5K genes, PIP5K7, PIP5K8, and PIP5K9, constitute a subgroup highly conserved in land plants, suggesting that they retain a critical function shared by land plants. In this study, we comprehensively investigated the biological functions of the PIP5K7–9 subgroup genes. Reporter gene analyses revealed their preferential expression in meristematic and vascular tissues. Their YFP-fusion proteins localized primarily to the plasma membrane in root meristem epidermal cells. We selected a mutant line that was considered to be null for each gene. Under normal growth conditions, neither single mutants nor multiple mutants of any combination exhibited noticeable phenotypic changes. However, stress conditions with mannitol or NaCl suppressed main root growth and reduced proximal root meristem size to a greater extent in the pip5k7pip5k8pip5k9 triple mutant than in the wild type. In root meristem epidermal cells of the triple mutant, where plasma membrane localization of the PtdIns(4,5)P₂ marker P24Y is impaired to a large extent, brefeldin A body formation is retarded compared with the wild type under hyperosmotic stress. These results indicate that PIP5K7, PIP5K8, and PIP5K9 are not required under normal growth conditions, but are redundantly involved in root growth adaptation to hyperosmotic conditions, possibly through the PtdIns(4,5)P₂ function promoting plasma membrane recycling in root meristem cells.     

Growth of seedling roots in response to external osmotic stress by polyethylene glycol 20,000

Seedling roots of ten plant species were grown in siliceous sand wetted with solutions of polyethylene glycol (PEG) of MW=20,000 with osmotic potentials of 0.0, ? 0.25, ? 0.5 and ? 1.0 MPa. After 48 h growth under controlled lighting, root elongation and root diameter were measured. Root elongation of all species was reduced by increasing levels of external osmotic stress. Dicotyledonous species were affected more than monocotyledons at potentials of ? 0.25 and ? 0.5 MPa but less at ? 1.0 MPa. Root diameters of all the species were thicker than those of the unstressed at potentials of ? 0.25 and ? 0.5 MPa. At a potential of ? 1.0 MPa the dicotyledons were still thicker, though not by as much as they were at ? 0.25 and ? 0.5 MPa. The monocotyledons, in contrast, were thinner at ? 1.0 MPa. There was a significant positive correlation (r=0.81, p <0.01) between root diameter and root elongation at ? 1.0 MPa potential. Species were ranked according to the relative root elongation (RRE) and relative root thickness (RRT) at the highest level of stress (? 1.0 MPa). In both rankings dicotyledonous species were in the top ranks and monocotyledous species were in lower positions. The results are compared with those for the elongation and thickening of roots growing against external mechanical stress obtained in a previous study. There were good correlations between the responses observed for the two types of external stress. The implications of these findings are discussed.     

Free and conjugated desmethylsterol composition of Zea mays hybrids exposed to mild osmotic stress

Five hybrids of Zea mays (A619 × H60, B73 × MO17, B73 × PA91, B73 × VA17. A632 × H96) were grown hydroponically at osmotic potentials of -0.1 (control), -0.4 and -0.6 MPa. Dry weights of hybrids B73 × VA17 and A632 × H96 decreased significantly at -0.4 and -0.6 MPa. Few changes were observed in the free sterol, steryl glycoside and steryl ester fractions. However, steryl esters of the roots and stems of osmotically treated plants declined in most of the hybrids. The stigmasterol to sitosterol ratio increased in all three steryl fractions in the roots of B73×VA17 and A632×H96 as a result of the osmotic treatments. B73×VA17 and A632×H96 also exhibited the greatest initial electrolyte leakage when leaf dises were subjected to osmotic stress. A632×H96 and possibly B73×VA17 were less tolerant to osmotic treatments than the other hybrids. Modification of sterols may reflect initial events that lead to the stabilization or destabilization of cellular membrances which in turn may affect the tolerance of plants to stressful environments.     

Photosynthetic gas exchange response of poplars to steady-state and dynamic light environments

The steady-state and dynamic photosynthetic response of two poplar species (Populus tremuloides and P. fremontii) to variations in photon flux density (PFD) were observed with a field portable gas exchange system. These poplars were shown to be very shade intolerant with high light saturation (800 to 1300 mol photons m^–2 s^–1) and light compensation (70 to 100 mol m^–2 s^–1) points. Understory poplar leaves showed no physiological acclimation to understory light environments. These plants become photosynthetically induced quickly (10 min). Activation of Rubisco was the primary limitation for induction, with stomatal opening playing only a minor role. Leaves maintained high stomatal conductances and stomata were unresponsive to variations in PFD. Leaves were very efficient at utilizing rapidly fluctuating light environments similar to those naturally occurring in canopies. Post-illumination CO₂ fixation contributed proportionally more to the carbon gain of leaves during short frequent lightflecks than longer less frequent ones. The benefits of a more dynamic understory light environment for the carbon economy of these species are discussed.     

Photosynthetic response of three climber plant species to osmotic stress induced by polyethylene glycol (PEG) 6000

We examined the photosynthesis response to osmotic stress in three climber plant species, Pharbitis nil (Linn.) Choisy, Lonicera japonica Thunb, and Parthenocissus tricuspidata (Sieb.et Zucc.) Planch. All climber plants were exposed to osmotic stress induced by polyethylene glycol (PEG) 6000 at 4 levels (slight, moderate, severe osmotic and the control) for 30?days. Photosynthesis response was determined by measuring leaf photosynthesis, chlorophyll fluorescence, carbonic anhydrase activity and stable carbon isotope ratios. P. nil maintained high photosynthetic activity under long-term moderate osmotic stress due to both stable photosystem II photochemical efficiency and high carbonic anhydrase activity. L. japonica maintained high photosynthetic activity under long-term moderate stress due to high carbonic anhydrase activity rather than photosystem II photochemical efficiency. P. tricuspidata tolerated only short-term moderate osmotic stress and long-term slight osmotic stress because its response was mainly stomatal limitation, with the lowest photosynthetic activity and hardly any carbonic anhydrase activity. Carbonic anhydrase activity was inversely correlated with stable carbon isotope ratios. The regulation by carbonic anhydrase was probably the reason for P. nil and L. japonica to tolerate long-term moderate osmotic stress. The selection on the species should consider the differential adaptation mechanism to osmotic stress during the development of drought-resistant plants.     

Comparison of polyethylene glycol 3350 induced osmotic stress and soil drying for drought simulation in three woody species

 Drought simulation usually involves either soil drying or the use of an osmoticum, such as high molecular weight (>3000) polyethylene glycol (PEG). Although easy to apply, PEG absorption and toxicity remain a concern. This study compared the effects of soil drying and use of an osmoticum (PEG 3350). Osmotic stress and soil drought were applied to 5-month-old seedlings of jack pine (Pinus banksiana Lamb.) and black spruce [Picea mariana (Mill) B.S.P.] , which are both coniferous species from cold, boreal regions of North America, and flooded gum (Eucalyptus grandis W. Hill ex Maiden), a hardwood species growing in warmer, sub-tropical regions of Australia. Results showed that PEG 3350 was absorbed by roots, transported to shoots, and deposited on the leaves of both flooded gum and jack pine (but not black spruce). PEG lowered relative water content and damaged leaf tissues in both species, and also damaged stomata of flooded gum. Although 12 days of PEG-induced osmotic stress produced a decline in water potentials that was similiar to soil drying, it also caused significantly higher membrane injury and reduced net photosynthesis and stomatal conductance in leaves of all three species. Recovery of net photosynthesis and stomatal conductance in PEG-treated jack pine and black spruce was also slower after stress alleviation. Even a short exposure to PEG 3350 adversely affected seedlings compared to soil drought. These results confirmed that drought effects may vary, depending on the species and the method of stress induction. Received: 6 March 1996 / Accepted: 17 September 1996     

Effect of NaCl and isoosmotic polyethylene glycol stress on gas exchange in shoots of the C4 xerohalophyte Haloxylon aphyllum (Chenopodiaceae)

The effects of NaCl (200 mM) and osmotic stress generated by polyethylene glycol (PEG) on PSII maximal quantum efficiency, photosynthetic CO₂/H₂O gas exchange at two CO₂ concentrations, content of chlorophyll, proline, and malondialdehyde were investigated in shoots of C₄ xerohalophyte Haloxylon aphyllum (Chenopodiaceae). The PEG treatment induced a low water osmotic potential (?0.4 MPa) and inhibited photosynthesis (by a factor of 2) and transpiration (by a factor of 4). The NaCl treatment, at equal osmoticity conditions, reduced transpiration (by a factor of 2) and stimulated photosynthesis (by a factor of 2.5). Only the PEG-treated plants showed osmotic stress effects, which were demonstrated by an increase in proline and malondialdehyde contents in the shoot tissue. The data indicated that the halophilic character of this species was essential for maintaining the plant water status and photosynthesis under osmoticity induced by NaCl treatment. Herewith, the presence of C₄-type photosynthesis appeared to be just an auxiliary mechanism, because this xerohalophyte did not reveal the efficiency in water use typical for C₄ plants under osmotic stress, in the absence of a saline substrate.     

Leaf gas exchange and grain yield of common bean exposed to spermidine under water stress

Three prevalent aliphatic polyamines (PAs) include putrescine, spermidine, and spermine; they are low-molecular-mass polycations involved in many physiological processes in plants, especially, under stressful conditions. In this experiment, three bean (Phaseolus vulgaris L.) genotypes were subjected to well-watered conditions and two moderate and severe water-stressed conditions with and without spermidine foliar application. Water stress reduced leaf relative water content (RWC), chlorophyll contents, stomatal conductance (gs), intercellular CO₂ concentration (C_i), transpiration rate, maximal quantum yield of PSII (F_v/F_m), net photosynthetic rate (P_N), and finally grain yield of bean plants. However, spermidine application elevated RWC, gs, C_i, F_v/F_m, and P_N, which caused an increase in the grain yield and harvest index of bean plants under water stress. Overall, exogenous spermidine could be utilized to alleviate water stress through protection of photosynthetic pigments, increase of proline and carotenoid contents, and reduction of malondialdehyde content.

     

Assessment of changes in growth traits,oxidative stress parameters,and enzymatic and non-enzymatic antioxidant defense mechanisms in Lepidium draba plant under osmotic stress induced by polyethylene glycol

Protoplasma - Lepidium draba is a weed with the medicinal properties which few researches have been done on it. In this study, some traits, related to the osmotic stress, in 14-day-old L. draba...     

Polyamine concentrations and arginine decarboxylase activity in wheat exposed to osmotic stress

The activity of L-arginine decarboxylase (ADC: EC 4.1.1.19)and polyamine content were examine in intact wheat plants ( Triticum aestivum L. cv. Sappo) exposed to osmotic stress (0.4 M mannitol) for 5 days. ADC activity was increased in first and second leaves and in roots of mannitol-stressed plants. Concentrations of putrescine, cadaverine and spermine were generally increased in leaves and roots of plants exposed to mannitol, whereas spermidine was reduced in first leaves and roots of these plants. In an attempt to determine the localization of mannitol in stressed wheat. ¹⁴C-mannitol was fed to plants grown in liquid culture. Most of the mannitol was detected in roots (84%), while small amounts were found in first (9%) and second (7%) leves.
Since it seemed possible that some of the effects on polyamine metabolism caused by exposure to mannitol could have been the result of water stress. polyamine metabolism was also studied in plants water stressed by exposure to 2% polyethylene glycol (PEG) 4000. ADC activity was not altered by exposure to PEG. but concentrations of putrescine, spermidine and spermine were generally reduced in leaves and roots of stressed plants. Cadaverine concentrations were not significantly affected by exposure to PEG. Spermidine and spermine concentrations were reduced in first and second leaves but remained unchanged in roots of plants exposed to PEG.     

Arabidopsis eIF5A3 influences growth and the response to osmotic and nutrient stress

AteIF5A3, one of three genes encoding eukaryotic translation initiation factor 5A (eIF5A) in Arabidopsis thaliana, and corresponding genes PdeIF5A3 from Populus deltoides (eastern cottonwood) and SleIF5A4 from Solanum lycopersicum (tomato) were constitutively over‐expressed in A. thaliana. The resultant transgenic plants exhibited enhanced vegetative and reproductive growth. Indeed, the increase in seed yield relative to empty vector controls for the PdeIF5A3 over‐expressing plants ranged from 50% to 300% depending on the line. The PdeIF5A3 over‐expressing plants also exhibited enhanced fitness when exposed to osmotic and nutrient (N, P and K) stress. The spatial localization of AteIF5A3 was visualized by confocal microscopy using transgenic plants expressing P_AteIF5A3:GFP‐AteIF5A3. GFP fluorescence reflecting expression of AteIF5A3 was detectable in the phloem, particularly companion cells, of roots, stems and leaves, in the epidermal cells of the root tip, in the columella cells of the root cap and in the chalazal tissue of fertilized ovules, which all play a pivotal role in nutrient or hormone translocation. Thus, AteIF5A3 appears to be involved in supporting growth and to play a regulatory role in the response of plants to sub‐lethal osmotic and nutrient stress.     

Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free‐air CO2 enrichment†

Poplar (Populus × euroamericana) saplings were grown in the field to study the changes of photosynthesis and isoprene emission with leaf ontogeny in response to free air carbon dioxide enrichment (FACE) and soil nutrient availability. Plants growing in elevated [CO₂] produced more leaves than those in ambient [CO₂]. The rate of leaf expansion was measured by comparing leaves along the plant profile. Leaf expansion and nitrogen concentration per unit of leaf area was similar between nutrient treatment, and this led to similar source–sink functional balance. Consequently, soil nutrient availability did not cause downward acclimation of photosynthetic capacity in elevated [CO₂] and did not affect isoprene synthesis. Photosynthesis assessed in growth [CO₂] was higher in plants growing in elevated than in ambient [CO₂]. After normalizing for the different number of leaves over the profile, maximal photosynthesis was reached and started to decline earlier in elevated than in ambient [CO₂]. This may indicate a [CO₂]‐driven acceleration of leaf maturity and senescence. Isoprene emission was adversely affected by elevated [CO₂]. When measured on the different leaves of the profile, isoprene peak emission was higher and was reached earlier in ambient than in elevated [CO₂]. However, a larger number of leaves was emitting isoprene in plant growing in elevated [CO₂]. When integrating over the plant profile, emissions in the two [CO₂] levels were not different. Normalization as for photosynthesis showed that profiles of isoprene emission were remarkably similar in the two [CO₂] levels, with peak emissions at the centre of the profile. Only the rate of increase of the emission of young leaves may have been faster in elevated than in ambient [CO₂]. Our results indicate that elevated [CO₂] may overall have a limited effect on isoprene emission from young seedlings and that plants generally regulate the emission to reach the maximum at the centre of the leaf profile, irrespective of the total leaf number. In comparison with leaf expansion and photosynthesis, isoprene showed marked and repeatable differences among leaves of the profile and may therefore be a useful trait to accurately monitor changes of leaf ontogeny as a consequence of elevated [CO₂].     

根系渗透胁迫时杨树光合作用光抑制与活性氧的关系

为更多地了解自然条件下活体叶片的光抑制,研究了渗透胁迫时杨树无性系幼苗叶片的光抑制与活性氧代谢的关系．结果表明,随胁迫时间的延长和胁迫强度的增大,杨树叶片O2^-生成加快,H2O2和丙二醛(MDA)含量增多,超氧物歧化酶(SOD)活性升高,过氧化氢酶(CAT)和抗坏血酸过氧化物酶(APX)活性降低,活性氧代谢失衡,光合作用的光抑制加剧．用二乙基二硫代氨基甲酸铜盐抑制SOD活性,或用甲基紫精加速O2^-的生成,亦可使杨树叶片发生光抑制．渗透胁迫时杨树无性系幼苗清除H2O2能力降低,限制了叶片通过Mehler反应耗散过剩光能,防御光破坏作用的发挥;光抑制的发生与活性氧的积累有关．     

Osmotic adjustment in Lycopersicon esculentum and L. Pennellii under NaCl and polyethylene glycol 6000 iso–osmotic stresses

Changes in leaf solute contents in response to saline (NaCl) and osmotic (polyethylene glycol, PEG, 6000) stresses were measured in three different salt tolerant cultivars of Lycopersicon esculentum (L.) Mill. (Pera, P-73 and Volgogradskij), and its wild relative L. pennellii (Correll) D'Arcy accession PE-47. Iso-osmotic stresses (–0. 5 MPa) of NaCl (140 mM) and PEG 6000 (150 g l^-1) were applied to one-month old plants for 3 weeks. Decreasing leaf dry weight was similar in L. pennellii or L. esculentum cv. P-73 and Volgogradskij under both stresses, while leaf dry weight of L. esculentum cv. Pera decreased more under PEG stress than under NaCl stress. Water contents decreased in all the PEG treated populations, while their calculated solute potential (Ψ_s increased. Under osmotic stress, the total ion contents decreased in relation to control, whereas organic solutes (sugars, amino acids and organic acids) markedly increased in both tomato species, specially in the tomato cultivars, where these solutes represented 50% of the Ψ₅ calculated. Soluble sugar increase was three times higher in leaves of L. esculentum than in the leaves of L. pennellii. Free proline increased under both stresses and its content was highest in L. esculentum and in L. pennellii, respectively, under NaCl and PEG stresses. Nevertheless, the contribution of this metabolite to Ψ_s did not exceed 5%, irrespective of treatment and species. The greater organic solute accumulation in L. esculentum than in L. pennellii– which was not reflected in their Ψ₅ values – was not correlated with the tolerances of the two species to osmotic stress. Therefore, osmotic adjustment may not be the only process influencing salt and drought tolerances in tomato; the ability of plants to regulate their metabolic and physiological functions could also play an important role under these harmful conditions. The possible roles of inorganic solutes and metabolites in osmotic adjustment, energetic metabolism and redox regulation are discussed